The present invention relates to a seismic isolator for protecting a structure against damage or destruction caused by earthquake vibrations, i.e., for isolating seismic vibrations to be transmitted to the structure and, more particularly, to a seismic isolator having a seismic isolating function which varies in accordance with the scale of an earthquake.
Various types of conventional seismic isolators are used in large structures such as buildings to prevent the buildings from damage or destruction caused by earthquake vibrations.
FIGS. 1 and 2 respectively show conventional seismic isolators. A plurality of seismic isloators each shown in FIG. 1 is inserted between a building 1 and its foundation 2 to support the weight of the building 1. When an earthquake occurs, the vibrations are reduced by the seismic isolators. The reduced vibrations are transmitted to the building 1, thereby protecting the building from damage or destruction. The seismic isolators shown in FIG. 1 comprises a support base 3 fixed on the foundation 2 and a support member 4 disposed between the support base 3 and the lower surface of the building 1. More specifically, the support member 4 comprises a lower end plate 5 fixed on the support base 3, an upper end plate 6 fixed on the lower surface of the building 1, and an elastic member 7 disposed between the lower and upper end plates 5 and 6 which is made of a vibration-proof material such as rubber vibration isolator or laminated rubber bearing. The elastic member 7 provides the building 1 with horizontal flexibility.
According to the seismic isolator shown in FIG. 1, when an earthquake occurs and its vibrations are transmitted from the foundation 2 to the building 1 through the support member 4, part of vibration energy of the earthquake is converted to deformation energy to be stored in the elastic member 7 since the elastic member 7 in the support member 4 is deformed horizontally due to earthquake vibrations. Therefore, the vibration energy of the earthquake is barely transmitted to the building 1, thereby improving the seismic proofing of the building 1.
These physical phenomenas means the vibration characteristics as follows. In the seismic isolators shown in FIG. 1, the primary natural frequency of the entire structure including the building 1 and the seismic isolators is sufficiently smaller than the natural frequency of the building 1 itself, thereby protecting the building 1 from resonant response.
However, in the seismic isolator shown in FIG. 1, the vibration energy of the earthquake is stored by only the elastic member 7 in the support member 4. The vibration energy component stored by the elastic member 7 upon its deformation is relatively small. As a result, the building 1 may not be effectively isolated by the seismic isolator shown in FIG. 1 for a medium scale earthquake (VII&lt;M&lt;VIII where M is the Modified Mercalli Intensity Scale). This is because in this case the deformation of the elastic member 7 in the support member 4 exceeds its allowance and so the elastic member 7 will be destroyed when an earthquake greater than a medium scale one occurs. Therefore, when such an earthquake occurs, the building 1 may be damaged or destroyed.
However, it is absolutely vital for some structures to be completely protected from earthquakes, irrespective of the scale of the earthquake. A typical example is a reactor building in a nuclear power station.
An seismic isolator used in a building such as a reactor building is illustrated in FIG. 2. The seismic isolator shown in FIG. 2 has substantially the same construction as that in FIG. 1. The same reference numerals in FIG. 2 denote the same parts as in FIG. 1, and a detailed description thereof will be omitted. A description will be made only of the different components.
The seismic isolation shown in FIG. 2 has a sliding plate 8 fixed on the lower surface of a building 1. The lower surface of the sliding plate 8 serves as a sliding surface. The upper surface of the upper end plate 6 in a support member 4 also serves as a sliding surface. The sliding surface of the upper end plate 6 is brought into slidable contact with that of the sliding plate 8.
When a small earthquake occurs, the vibration energy of the earthquake which would otherwise be transmitted to the building 1 can be stored by deformation of an elastic member 7 in the support member 4 in the same manner as the seismic isolator shown in FIG. 1. When an earthquake of more than a predetermined scale occurs, i.e., when a horizontal force acting on the sliding plate 8 and hence the building 1 exceeds a frictional force (corresponding to a product of a friction coefficient of the sliding surface of the sliding plate 8 and a weight imposed on the sliding plate 8 of the seismic isolator), the sliding plate 8 and hence the building 1 slides on the upper end plate 6. While the sliding plate 8 is sliding on the upper end plate 6, a force exceeding the frictional force will not be transmitted to the building 1 irrespective of the scale of the earthquake, and the acceleration of the lower portion of the building 1 will not exceed a product of the friction coefficient and the gravitational acceleration. In this case, when the building 1 starts sliding on the upper end plate 6 through the sliding plate 8, the vibration energy of the earthquake which can be spent by the seismic isolator shown in FIG. 2 corresponds to a product of a sliding displacement of the building 1 and the frictional force.
According to the seismic isolator in FIG. 2, therefore, when a relatively small earthquake occurs, part of the vibration energy of the earthquake which would otherwise be transmitted to the building 1 can be stored by the elastic member 7 in the support member 4. In addition, when a relatively large earthquake occurs, the building 1 itself is horizontally shifted such that the sliding plate 8 slides on the upper end plate 6 in the support member 4, thereby preventing excessive vibration energy from an earthquake having more than a predetermined value from being transmitted to the building 1.
In the seismic isolator shown in FIG. 2, the relationship between a displacement .delta. of the building 1 with respect to the foundation 2 and an earthquake force F transmitted to the building 1 is illustrated in FIG. 3 when the building 1 is vibrated by an earthquake at a constant amplitude. Referring to FIG. 3, line segment A.sub.0 indicates a deformation state of the support member 4 immediately after the earthquake vibration is transmitted to the building 1, line segment B.sub.0 indicates a deformation state of the support member 4 when the building 1 slides, line segment c.sub.0 indicates a deformation state of the support member 4 toward a direction opposite the direction of the previous state thereof, line segment D.sub.0 indicates a sliding state of the building 1 toward a direction opposite that of the previous state thereof, and line segment E.sub.0 indicates a deformation state of the support member 4 toward the direction of the previous state. The region surrounded by the line segments B.sub.0, C.sub.0, D.sub.0, and E.sub.0 excluding the line segment A.sub.0 indicates the earthquake vibrations energy to be spent by a cycle of sliding when the building 1 slides on the upper end plate 6 through the sliding plate 8. However, when a magnitude or scale of the earthquake does not cause sliding of the building 1, the relationship between the earthquake force acting on the building 1 and the displacement of the building 1 is represented by line segment A.sub.0 and broken line segment A.sub.0 '.
As is apparent from the above description according to the seismic isolator shown in FIG. 2, when an earthquake is of more than a predetermined scale, the building 1 is displaced such that the sliding plate 8 slides on the upper end plate 6 in the support member 4, and a force exceeding the force F.sub.0 will not act on the building 1 irrespective of the scale of the earthquake, as is apparent from FIG. 3. In the seismic isolator shown in FIG. 2, even in the case of a strong earthquake having a large scale (M&lt;IX where M is the modified Mercalli Intensity Scale), the building 1 will not be damaged or destroyed.
Indeed the seismic isolator (FIG. 2) has the above advantages, but it also has the following drawback. With this seismic isolator it is difficult to determine how large an earthquake may slide the building 1. If the seismic isolator is so designed as to cause the building 1 to slide when an earthquake of medium scale or a greater scale hits the building 1, its seismic isolation effect is achieved by only the deformation of the elastic member 7, which stores the vibration energy of the earthquake, when an earthquake smaller than the medium scale earthquake occurs. Consequently, in this condition, the vibration energy of the earthquake is not effectively spent by the seismic isolator of FIG. 2. Thus, this seismic isolator has the same disadvantage as the apparatus of FIG. 1.
For this reason, the seismic isolator (FIG. 2) must be so designed as to cause the building 1 to slide when an earthquake smaller than the medium scale earthquake occurs. Once the building 1 slides, the building 1 will not always return to the initial position even when the earthquake has finished. In other words, it is quite possible that the building 1 is displaced with respect to the foundation 2 when the earthquake has finished. Therefore, as the building 1 is displaced from the initial position, large-scale repair must be performed to set the building 1 back in the initial position.
In addition, it seems that earthquakes of medium scale frequently occur in the district, that is, high seismic zone, where the structures with the seismic isolation are built. Every time the medium scale earthquake occurs, the building 1 must be repaired, resulting in a reduction in utility of the system including the building 1, and in high repair costs.